D-orbital Splitting Research Articles

Molecular Origins of Near-Infrared Luminescence in Molybdenum and Tungsten Oxyhalide Perovskites.

Materials with near-infrared (near-IR) luminescence are desirable for applications in communications and sensing, as well as biomedical diagnostics and imaging. The most used inorganic near-IR emitters rely on precise doping of host crystal structures with select rare-earth or transition metal ions. Recently, another class of materials with intrinsic near-IR emission has been reported. The compositions of these materials were initially described as vacancy-ordered halide double perovskites Cs2MoCl6 and Cs2WCl6, but further investigation by some of us on the compound reported as Cs2WCl6 revealed an oxyhalide instead, with a composition Cs2WO x Cl6-x , where 1 < x < 2. Here we demonstrate that the Mo compounds similarly possess the composition Cs2MoO x Cl6-x or Cs2MoO x Br6-x where 1 < x < 2. Preparing the pure halide appears harder for Mo than for W, and we have not succeeded in doing so. The distinctly different composition requires the coordination environment and oxidation state for the Mo and W centers to be reconsidered from what was assumed for the pure halides. In this work, we examine the mechanism for near-IR emission in these materials given their true structures and compositions. We demonstrate that the luminescence is due to the specific d-orbital splitting caused by the presence of oxygen in the distorted [MOX5]2- octahedra (X is Cl or Br). The fine structure in the emission spectra at low temperatures has been resolved and is attributed to vibronic coupling to the Mo-O and W-O bond stretches. Understanding the true structure and composition of these interesting materials, besides explaining the near-IR luminescence, suggests how this desirable emission can be realized and manipulated.

Theoretical and Experimental Insights of Magnesium-doped Cobalt Ferrites for Supercapacitor Applications

In the present study, we synthesize magnesium-doped cobalt ferrite (Co0.75Mg0.25Fe2O4) nanoparticles as stable and high-performance electrodes for supercapacitor application via the sol-gel combustion method and anneal them at 200°C (MCF1), 300°C (MCF2), and 500°C (MCF3). Experimental and density functional theory (DFT) simulations of Co0.75Mg0.25Fe2O4 nanoparticles affirm a normal spinel cubic structure with partial tetrahedral occupancy and lattice expansion (8.3524 to 8.3704 Å) with subsequent temperature rise. The partial occupancies curtail the charge diffusion path length and favor a pronounced energy storage capacity. Morphological assessment also verifies the clustered and homogenous normal spinel cubic structure. Such cationic ordering increases the electronic bandgap (0.84 eV → 1.57 eV) with d-orbital splitting due to crystal field effects. However, a non-linear impact upon the optical bandgap (1.214 eV to 1.212 eV (MCF1 to MCF2) and increases to 1.22 eV (MCF3)) is the consequence of bond dissociations favoring adequate e-h charge separations at elevated temperatures. Our experimental outcomes reveal a high specific capacitance of 579.3 Fg−1 for MCF3 at 0.5 Ag−1 with a cyclic stability of 96.49% after 1000 cycles at 5 Ag−1.

Unveiling enhanced electron-mediated peroxymonosulfate activation for degradation of emerging organic pollutants

The electron transfer pathway activated by peroxymonosulfate (PMS) has garnered significant attention for the removal of emerging organic pollutants from water. However, overcoming the two-step energy barriers involved in electron transport across reaction interfaces presents a formidable challenge. To surmount the two-step energy barriers of pollutant-catalyst and catalyst-PMS, a catalyst with nano-island structure was developed, forming efficient ternary catalytic interfaces with PMS and pollutants. The presence of atomic pairs within the catalyst punched through the electron transport channels, resulting in a pseudo-first-order kinetic rate of 2.06 min−1 for bisphenol A, which was 5.1 times higher than that of the control sample. The electronic coupling of atomic pairs exerted a profound impact on the splitting of d-orbitals, effectively elevating the d-orbital unoccupancy and reducing the two-step energy barriers encountered by electrons from pollutants to catalysts, and subsequently to PMS, which promoted efficient degradation of pollutants. Furthermore, through the association of two-step energy barrier, the feasibility of utilizing the orbital interaction as a descriptor of the degradation rate by electron-mediated PMS activation was demonstrated. Additionally, the intermediates generated through electron transfer pathway exhibited a lower risk of bioaccumulation. This work inspires insights into the electron-mediated mechanism of multinary catalytic interface reactions.

Establishing an orbital-level understanding of active origins of heteroatom-coordinated single-atom catalysts: The case of N2 reduction

Heteroatom-coordinated single-atom catalysts (SACs) supported by porous graphene exhibit high activity in electrochemical reduction reactions. However, the underlying active origins are complex and puzzling, hindering the development of efficient catalysts. Herein, we investigate the active origins of heteroatom-coordinated Fe-XmYn SACs (X, Y = B, C, N, O, m + n = 4) toward nitrogen reduction reaction (NRR) as a model reaction, through comprehensive analysis of structural, energetic, and electronic parameters. Specifically, the number and arrangement of heteroatoms are found to significantly affect the degree of d-orbital splitting and magnetic moment of the Fe center. Moreover, d-orbital splitting energy (dSE), rather than the conventional d-band theory, explains the adsorption behavior of intermediates in multi-step electron–proton coupling (EPC) reactions. In addition, both s- and d-orbitals of Fe are found to be important for Fe–N bonding, which promotes charge transfer (CT) and N2 activation. Importantly, CT is thought to influence the Pauli repulsion and orbital interaction. Correspondingly, relationships are unveiled between limiting potential (Ulimit) and adsorption energy ΔE(*NNH), dSE, CT, Fe–N bond. In all, this work provides orbital-level insights into the active origins of Fe-XmYn SACs, contributing to the understanding of intrinsic mechanism and the design of electrocatalysts for multi-step EPC reactions.

Coordination-Controlled Catalytic Activity of Cobalt Oxides for Ozone Decomposition.

Nowadays, it is still elusive and challenging to discover the active sites of cobalt (Co) cations in different coordination structures, though Co-based oxides show their great potency in catalytic ozone elimination for air cleaning. Herein, different Co-based oxides are controllably synthesized including hexagonal wurtzite CoO-W with Co2+ in tetrahedral coordination (CoTd2+) and CoAl spinel with dominant CoTd2+, cubic rock salt CoO-R with Co2+ in octahedral coordination (CoOh2+), MgCo spinel with dominant Co3+ in octahedral coordination (CoOh3+), and Co3O4 with mixed CoTd2+ and CoOh3+. The valences are proved by X-ray photoelectron spectroscopy, and the coordinations are verified by X-ray absorption fine structure analysis. The ozone decomposition performances are CoOh3+ ∼ CoOh2+ ≫ CoTd2+, and CoOh3+ and CoOh2+ show a lower apparent activation energy of ∼42-44 kJ/mol than CoTd2+ (∼55 kJ/mol). In specific, MgCo shows the highest decomposition efficiency of 95% toward 100 ppm ozone at a high space velocity of 1,200,000 mL/gh, which still retains at 80% after a long-term running of 36 h at room temperature. The high activity is explained by the d-orbital splitting in the octahedral coordination, favoring the electron transfer in ozone decomposition reactions, which is also verified by the simulation. These results show the promising prospect of the coordination tuning of Co-based oxides for highly active ozone decomposition catalysts.

Tailoring the d-Orbital Splitting Manner of Single Atomic Sites for Enhanced Oxygen Reduction.

Regulating the electronic states of single atomic sites around the Fermi level remains a major concern for boosting the electrocatalytic oxygen reduction reaction (ORR). Herein, a Fe d-orbital splitting manner modulation strategy by constructing axial coordination on FeN4 sites is presented. Experimental investigations and theoretical calculations reveal that the axial tractions induce the distortion of square-planar field (FeN4 SP), up to the quasi-octahedral coordination (FeN4 O1 OCquasi ), thus leading to the electron rearrangement with a diluted spin polarization. The declined population of unpaired electrons in dz 2 , dx z and dyz states engenders a moderate adsorption of ORR intermediates, thereby reinforcing the intrinsic reaction activity. In situ infrared spectroscopy further demonstrates that the reordering of d-orbital splitting and occupation facilitates the desorption of *OH. The FeN4 O1 OCquasi exhibits a dramatic improvement of kinetic current density and turnover frequency, which are fivefold and tenfold higher than those of FeN4 SP. This work presents a novel understanding on improving the electrocatalytic performance through the orbital-scale manipulation.

Spin-state energetics and magnetic anisotropy in penta-coordinated Fe(III) complexes with different axial and equatorial ligand environments.

The penta-coordinated trigonal-bi-pyramidal (TBP) Fe(III) complex (PMe2Ph)2FeCl3 shows a reduced magnetic anisotropy in its intermediate-spin (IS) state as compared to its methyl-analog (PMe3)2Fe(III)Cl3. In this work, the ligand environment in (PMe2Ph)2FeCl3 is systematically altered by replacing the axial -P with -N and -As, the equatorial -Cl with other halides, and the axial methyl group with an acetyl group. This has resulted in a series of Fe(III) TBP complexes modelled in their IS and high-spin (HS) states. Lighter ligands -N and -F stabilize the complex in the HS state, while the magnetically anisotropic IS state is stabilized by -P and -As at the axial site, and -Cl, -Br, and -I at the equatorial site. Larger magnetic anisotropies appear for complexes with nearly degenerate ground electronic states that are well separated from the higher excited states. This requirement, largely controlled by the d-orbital splitting pattern due to the changing ligand field, is achieved with a certain combination of axial and equatorial ligands, such as -P and -Br, -As and -Br, and -As and -I. In most cases, the acetyl group at the axial site enhances the magnetic anisotropy compared to its methyl counterpart. In contrast, the presence of -I at the equatorial site compromises the uniaxial type of anisotropy of the Fe(III) complex leading to an enhanced rate of quantum tunneling of magnetization.

Synthesis and molecular structural studies of racemic chiral-at-vanadium(V) complexes using an unsymmetric achiral phenolic bidentate ligand.

Mononuclear oxovanadium(V) complexes [V(O)XL (1: X = Ot-Bu, 2: X = Cl)] [H2L: 2,2'-methylene bis(4,6-di-tert-butylphenol)(4'-tert-butyl-6'-(1-adamantyl)phenol)] directed towards asymmetric catalysis have been synthesised as racemic compounds using an unsymmetric and achiral phenolic bidentate ligand (H2L), and NMR and UV-vis absorption spectroscopies, single-crystal X-ray diffraction, and IR spectroscopy revealed their racemic chiral-at-vanadium structures in solution and in the crystal. In addition, theoretical calculations revealed that the HOMO-LUMO energy gap is smaller for unsymmetric ligands, which promotes d-orbital splitting of the metal centre.

Influence of Number of Ligands and Point Group on the Electronic Structure of Co2+ Aqua-Complexes

The nucleation process of zeolitic imidazolate frameworks (ZIFs) is, to date, not yet completely understood, making the search for tailored materials very difficult. Recently, it has been shown that, during the formation process, the symmetry of the precursors is reduced by ligand elimination and substitution reactions. The octahedral precursors with simple ligands, such as water, methanol, and/or NO3-, are transformed to five- and finally four-coordinated complexes with imidazole ligands. This reduction of symmetry, caused both by the changing coordination environment and distortions from the perfect symmetry leading to another point group, will have a large influence on the electronic structure and more specifically on the d-orbital splitting. This, in turn, will affect the d-d electronic excitations, which can be followed using UV-vis spectroscopy and which can help to unravel the formation process. In this work, we systematically investigate how the lowering of the number of ligands affects the symmetry and thus the geometry and electronic structure of Co2+ complexes with six, five, and four aqua ligands. Therefore, we first resort to qualitative techniques, such as crystal field theory (CFT) and ligand field theory (LFT), which reveal that the orbital splitting is characteristic for the number of ligands. However, as these techniques are not capable of providing quantitative results without the use of experimental data as input, we perform various computational calculations. Both average of configuration (AOC) and unrestricted density functional theory (UDFT) are thoroughly investigated, and we will determine which technique is the best suited to properly describe the ground state of these systems. To investigate the dependency on the d-orbital occupation, we also investigated V2+, Mn2+, and Ni2+ hexa-aqua-complexes and compared them to the Co systems.

Predicting and understanding the structural stability, origin of half-metallic magnetic nature, and study of opto-electronic properties of Cs2KXCl6: X = Ti and V double perovskites

A succinct summary of various physical characteristics of halide double perovskites Cs2KXCl6: (X = Ti and V) has been summarized using Density Functional Theory (DFT), as applied in WIEN2k simulation code. These material’s stability in Fm-3 m cubic symmetry is confirmed by structural optimization, tolerance factor and elastic parameters. The electronic band structures and density of states calculated using GGA and GGA+mBJ show that these materials are semiconducting in spin down channels. The crystal field splitting of d-orbitals is used to explain the semiconducting ferromagnetic nature of these Ti- and V- based halide double perovskites. The ferromagnetic character of these double perovskites is suggested by the optimization and asymmetric DOS across Fermi level. We have also discussed about the origins of half-metallic bandgap and magnetism. The total magnetic moments of the Ti and V-based double perovskites are 1μB and 2μB, respectively, with the transition metal atoms (Ti/V) accounting for the major contributions. At the Fermi level, these double perovskites have 100% spin polarization, indicating that they could be used in spintronics.

Effective tuning of magnetic anisotropy in distorted pentagonal bipyramidal Ni(II) complexes via substitution of axial coligands.

A series of Ni(II) complexes with pyridine-based macrocyclic ligand L (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) with general formula [Ni(L)(X)2]0/2+ (X = Br- (1), I- (2), CH3CN (3), NCS- (4), imidazole (5)) was prepared and thoroughly investigated. X-ray molecular structures confirmed pentagonal bipyramidal geometry for all studied complexes with a strong Jahn-Teller distortion in the pentagonal equatorial plane and significantly elongated Ni-O distance(s) with a decrease of this distortion by varying axial coligands (CH3CN > Br- > I- > NCS- > imidazole). Direct current magnetic measurements revealed the easy-axis type of magnetic anisotropy with negative as well as positive axial zero-field-splitting parameter D ranging from +6.8 to -14.5 cm-1, which remains not affected in the halogenido series Cl- → Br- → I-, but which increases in the series with N-axial ligands in order CH3CN → NCS- → imidazole. Theoretical calculations helped to elucidate (i) the final coordination numbers 6 + 1 for 1 and 2, and 5 + 2 for 2-5, (ii) the pattern of splitting of d-orbitals, contributions of excited states to the final D-values and their final signs, and (iii) the complexity in the variation of the D and E parameters with elongation of axial bond distances in such strongly distorted systems. The studied complexes did not show any alternating magnetic susceptibility signal, but it was clearly documented that the magnetic anisotropy of the pentagonal bipyramidal Ni(II) complexes can be modulated/tuned by variation of axial coligands. Nevertheless, great care has to be taken for symmetry of the equatorial ligand field.

Probing the redox-conversion of Co(II)-disulfide to Co(III)-thiolate complexes: the effect of ligand-field strength.

The redox-conversion reaction of cobalt(II)-disulfide to cobalt(III)-thiolate complexes triggered by addition of the bidentate ligand 2,2'-bipyridine has been investigated. Reaction of the cobalt(II)-disulfide complex [Co2(L1SSL1)(X)4] (L1SSL1 = di-2-(bis(2-pyridylmethyl)amino)-ethyldisulfide; X = Cl or Br) [1X] with 2,2'-bipyridine (bpy) resulted in the formation of two different products, namely the cobalt(III)-thiolate complex [Co(L1S)(bpy)]X2 and the unexpected side product [Co2(L1SSL1)(bpy)2(X)2]X2. Crystals of [Co2(L1SSL1)(bpy)2(Cl)2](BPh4)2 [2Cl](BPh4)2 obtained after anion exchange showed the cobalt(II) ions to be in octahedral geometries with the nitrogen donors of the disulfide ligand arranged in a facial conformation and the chloride ion trans to the tertiary amine nitrogen. Remarkably, this side product cannot be converted to the cobalt(III)-thiolate compound [Co(L1S)(bpy)](SbF6)2 [3](SbF6)2 by removal of the chloride ion with use of a silver salt, as this causes scrambling of the ligands, resulting in the formation of [Co(bpy)3]n+. [Co(L1S)(bpy)](SbF6)2 was obtained in a pure form by addition of bpy to a solution in acetonitrile of the compound [Co(L1S)(MeCN)2]2+ [4]2+. Addition of NEt4Cl to [3](SbF6)2 regenerates the cobalt(II)-disulfide complex [1Cl] as confirmed spectroscopically. DFT studies revealed that the conversion from [1Cl] to [3]2+ most likely occurs via the hypothetical intermediate species [2Cl]2+mer, in which the nitrogen atoms of the disulfide ligand are arranged in a meridional conformation. Interestingly, the estimated d-orbital splitting energy of [3]2+ is lower than that of [4]2+, indicating that the ligand-field strength of bpy is lower than anticipated, which hampers clean conversion in the redox-conversion reaction. This study shows that the redox-conversion reaction between cobalt(II)-disulfide and cobalt(III)-thiolate complexes is intricate rather than straightforward.

Impurity level properties in transition metal doped α-Ga2O3 for optoelectronic applications

Doping engineering is necessary in the preparation of high-performance devices based on ultra-wide bandgap semiconductors. However, we do not yet understand the impurity level properties and their effects on performance modulation of transition metal doped α-Ga2O3. Here, using first-principles calculations with a hybrid functional, we find that IB and IIB transition metal dopants exhibit relatively lower formation energies with a theoretically high hole concentration compared with Mg dopant. However, the induced acceptor levels are so deep that the impurity levels are transformed into AX centers. Although such AX centers hinder the formation of p-type conductivity, they can increase the optical absorption, ranging from deep ultraviolet to infrared regions. For electron-rich doped α-Ga2O3, the donor levels go from deep to shallow, then turn into deep levels again and finally to relatively shallow again as the transition metal dopant goes from group IIIB to VIIIB of the periodic table. Because ionic bonds are formed between O and a dopant (IVB to VB) with a raised d-orbital, covalent characteristics are observed for the O dopant (VIB to VIIIB) bonds with evident d-orbital splitting. These results add new insights into the impurity levels of α-Ga2O3, but also reveal potential applications of transition metal dopants in α-Ga2O3.

Crystal and Electronic Structures of MoSi2-Type CrGe2 Synthesized under High Pressure.

Chromium germanides, namely, Nowotny chimney-ladder-phase CrGe1.77 and MoSi2-type CrGe2, were synthesized above 15 GPa or more via laser heating using a diamond anvil cell (LHDAC). MoSi2-type CrGe2, which is the most Ge-rich compound in the Cr-Ge system, crystallizes in the tetragonal structure with a space group of I4/mmm (no. 139) and lattice parameters of a = 3.24919(6) Å and c = 8.0523(3) Å and is isostructural with MoSi2. MoSi2-type CrGe2 has a deep pseudogap caused by the splitting of 3d orbitals with Cr, as evidenced by ab initio calculation. In this article, we have succeeded in synthesizing a binary compound between transition-metal and metalloid elements for the first time at high pressures above 10 GPa using the LHDAC. This pathway opens the possibility to explore more compounds in this system and may provide new insights into the fundamental interaction between these two elements.

Understanding the origin of semiconducting ferromagnetic character along with the high figure of merit in Cs2NaMCl6 (M = Cr, Fe) double perovskites

Thermoelectric devices have tendency to generate electric power by renovating waste heat. Semiconducting materials are the hunt for thermoelectric technology. We have traced the structural and mechanical stabilities, explored the electronic profile and thereafter unearthed the transport character of Cs2NaMCl6 (M = Cr, Fe) double perovskites. The electronic band structures and density of states from GGA and GGA + mBJ convey the semiconducting nature in both spin channels for these materials. The origin of semiconducting nature is explained with the help of crystal field splitting of d-orbitals. The optimization and asymmetric DOS across reference level suggest the ferromagnetic nature of double perovskites. The Boltztrap code with fixed relaxation time is used to determine the transport properties of Cs2NaMCl6 (M = Cr, Fe) double perovskites. The decreasing characteristic of Seebeck coefficient and resistance as a function of temperature suggests semiconductor character in these materials. The low thermal conductivity boosts the efficiency of materials to recover waste heat. The room temperature figure of merit (zT) is 0.71 and 0.79 for Cr and Fe-based double perovskites and with temperature, it increases. Thereby making these double perovskites suitable for waste heat harvesting. The results obtained while carrying out the investigation are in cohesion with available results in literature.

A simple rule of thumb for the explanation of d-orbital splitting in complexes

&lt;br /&gt;&lt;table class="t1" cellspacing="0" cellpadding="0"&gt;&lt;tbody&gt;&lt;tr&gt;&lt;td class="td1" valign="top"&gt;&lt;p class="p2"&gt;&lt;span class="s1"&gt; &lt;/span&gt;En las aulas de química a nivel preuniversitario y de ingeniería (disciplina no química) de primer año en universidades de la India, la división de los niveles de energía de los orbitales d en complejos es un concepto importante que debe aprenderse, pero no se explica explícitamente en los libros estándar usados. En los libros estándar como ‘Química inorgánica concisa’ de J .D. Lee y ‘Química inorgánica teórica’ de Marion Clyde Jr. Day y Joel Selbin, han explicado la división de los orbitales d en octaédricos, tetraédricos, planos cuadrados, etc. ., complejos muy bien. Lo mismo ocurre con el último libro de texto de química preuniversitario NCERT (Volumen I) escrito para la audiencia india. La razón por la que los niveles de energía de ciertos orbitales d están por encima del baricentro y por qué algunos están por debajo del baricentro, sin embargo, no se explica en ninguno de los libros (incluidos los últimos libros). Esta breve comunicación describe una simple regla empírica que permite explicar este fenómeno a los estudiantes. Además, se traza un gráfico importante en los libros estándar, pero no se explica la tendencia de la curva. Esta sencilla regla también es útil para explicar este gráfico y el fenómeno químico representado.&lt;span class="Apple-converted-space"&gt; &lt;/span&gt;&lt;/p&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/tbody&gt;&lt;/table&gt;

Investigating Complex Magnetic Anisotropy in a Co(II) Molecular Compound: A Charge Density and Correlated Ab Initio Electronic Structure Study.

Understanding magnetic anisotropy and specifically how to tailor it is crucial in the search for high-temperature single-ion magnets. Herein, we investigate the magnetic anisotropy in a six-coordinated cobalt(II) compound that has a complex geometry and distinct triaxial magnetic anisotropy from the perspective of the electronic structure, using electronic spectra, ab initio calculations, and an experimental charge density, of which the latter two provides insight into the d-orbital splitting. The analysis showed that the d-orbital splitting satisfactorily predicted the complex triaxial magnetic anisotropy exhibited by the compound. Furthermore, a novel method to directly compare the ab initio results and the d-orbital populations obtained from the experimental charge density was developed, while a topological analysis of the density provided insights into the metal-ligand bonding. This work thus further establishes the validity of using d-orbitals for predicting magnetic anisotropy in transition metal compounds while also pointing out the need for a more frequent usage of the term triaxial anisotropy in the field of single-molecule magnetism.

Chemical Bonding in Colossal Thermopower FeSb2.

FeSb2 exhibits a colossal Seebeck coefficient ( ) and a record-breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb2 are based on either ligand-field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb2 . Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X-ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single-electron Sb-Sb bond implied by the covalent model, and the large formal charge and expected d-orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences.

Predicting Oxygen Vacancy Polaron Size from D-Orbital Splitting in ABO3 (A=La, Sr; B=Mn, Fe, Co) Perovskites

Mixed Ionic Electronic Conducting (MIEC) transition metal oxide (ABO3) perovskites are utilized for catalytic,1 energy conversion,2,3 and other applications. The oxygen ionic conductivity of these MIECs depends on their oxygen vacancy concentration and ionic mobility. In turn, the oxygen ion concentration and mobility both depend on the charge of the B-site atom,4 which itself depends on factors such as the A site La/Sr ratio, the crystal structure, etc. Previous calculations demonstrated that the Fe atom charge state distribution observed in strontium ferrite (SrFeO3-δ) is caused by differences in the d-orbital splitting of octahedral (Oh) versus square pyramidal (SP) Fe-O coordination (Figure 1).5 This d-orbital splitting ensures that the two electrons left behind when a charge-neutral oxygen vacancy forms in cubic SrFeO3-δ are transferred to the second nearest Fe neighbors,5 resulting in a large oxygen vacancy polaron size that produces strong oxygen vacancy interactions at high oxygen nonstoichiometry. As a result, the oxygen vacancy formation energy increases with oxygen nonstoichiometry in cubic SrFeO3-δ.5 Previous calculations also demonstrated that in orthorhombic lanthanum ferrite (LaFeO3-δ) the formation of a neutral oxygen vacancy produces a polaron that is localized to the oxygen-vacancy-adjacent Fe atoms. Hence, in orthorhombic LaFeO3-δ the oxygen vacancies do not interact with increasing oxygen nonstoichiometry.6 The objective of the present work was to examine whether d-orbital splitting could also explain the oxygen vacancy behavior of other MIEC perovskites. Hence, the impact of d-orbital splitting on oxygen vacancy polaron size and formation energies in lanthanum strontium manganite and lanthanum strontium cobaltite were modeled for the first time. Here, the charge redistribution caused by oxygen vacancy formation was studied for six different compositions: LaFeO3-δ, SrFeO3-δ, LaMnO3-δ, SrMnO3-δ, LaCoO3-δ, and SrCoO3-δ. The GGA+U method with PAW potentials implemented in VASP was utilized for all the structural energy calculations. First, the Hubbard-U parameter was calibrated to describe the charge on the B-site atom (U = 3 was selected for Fe, as done previously).6 The oxygen vacancy formation energy as a function of oxygen non-stoichiometry (δ) was then calculated at 0 K in vacuum by varying the size of the supercell, with oxygen vacancy interactions being enabled by the periodic boundary conditions. The calculated polaron size for neutral-oxygen-vacancy–containing ABO3 structures are summarized in Table 1.In LaMnO3-δ, Mn3+ has an [Ar] 3d4 electronic configuration in Oh coordination. After the formation of a neutral oxygen vacancy, the two Mn atoms adjacent to the oxygen vacancy go from Oh to SP coordination and the excess electrons left behind by the removed oxygen are pushed to the a1g level of the second nearest neighboring Oh-Mn (instead of remaining localized to the b1g level of the SP-Fe). This long range charge transfer results in large polaron size. This behavior is in contrast with that of LaFeO3-δ. However, LaMnO3-δ and SrFeO3-δ exhibit comparable polaron sizes since Fe4+ has an [Ar] 3d4 electronic- configuration. In SrMnO3-δ, Mn4+ has a [Ar] 3d3 electronic configuration in Oh coordination. Therefore, the formation of a neutral oxygen vacancy localizes the electrons to the oxygen vacancy adjacent Mn atoms, producing a small polaron, as explained by the d-orgbial splitting in Figure 1. This d-orbital splitting analysis indicates that the polaron size in LSM > LSF > LSC. Additional work aimed at calculating the oxygen vacancy formation energies of other ABO3 compositions are in progress.

Thermoelectric Properties of 2D Ni3(HITP)2 and 3D Cu3(BTC)2 MOFs: First-Principles Studies

Metal-Organic Frameworks are nanoporous structures comprised of metal ions coordinated to electron-donating “linker” molecules. Although their low density and open structure give them low thermal conductivity, most MOFs are electrical insulators. Recently, however, we and others showed that electrically conducting frameworks can be synthesized by several routes, leading to both two- and three-dimensional materials with conductivities as high as 160 S/cm, suggesting potential for thermoelectric device applications. New 2D materials based on MOFs that resemble graphene in their topology are among those with the highest electrical conductivity. These materials, sometimes referred to as Metal-Organic Graphene Analogues (MOGs), display intriguing electronic, thermal, and optical properties, some of which depend on whether a monolayer or multilayer is used. For example, both metallic and semiconducting behavior are predicted, as well as quantum spin Hall and Z2 metallic states in monolayers. Only semiconducting behavior has been experimentally observed so far, however. In this presentation we describe experimental and theoretical studies of the fundamental charge transport processes controlling the electronic and thermal properties relevant to thermoelectric devices. In particular, we consider the thermoelectric properties of crystalline MOGs with the structure X3(HITP)2 in which X = Ni, Pd or Pt, and HITP = 2,3,6,7,10,11-hexaiminotriphenylene, calculated using ab initio simulations. The dependence of thermoelectric transport properties on the atomic structure are modeled by comparing the calculated band structure, band alignment, and electronic density of states of the three MOGs. We find that the thermoelectric transport properties depend strongly on both the interaction between the linkers and the metal ions and the d-orbital splitting of the metal ions induced by the linker crystal field. Moreover, a significant deviation from the Wiedemann-Franz law arises is predicted for p-type doping of these materials, with a nonlinear behavior of the Lorentz number as a function of hole concentration. The results predict that thermoelectric transport properties of X3(HITP)2 systems can be enhanced by replacing Ni(II) ions in the structure with heavy metal ions, thereby increasing the interaction between the metal ion and the ligands. We will also discuss a strategy for converting an insulating MOF to an electrically conducting one by inserting redox active molecules in the pores, which call Guest@MOF. The MOF Cu3(btc)2 (btc = benzenetricarboxylate), also known as HKUST-1, becomes electrically conducting when the molecule TCNQ (7,7,8,8-tetracyanoquinodimethane) is infiltrated into the ~1 nm pores of the MOF. TCNQ is a strong π acid that coordinates to unoccupied Cu(II) ions in the framework, generating new charge transfer bands in the UV-visible absorption spectrum. Thermoelectric measurements show that the majority charge carriers are holes, and a Seebeck coefficient of ~400 μV/K is obtained, more than double the value for Bi2Te3. Together, the results of this work provide fundamental guidance to optimize existing conducting MOFs and to design and discover new families of MOF-like materials for thermoelectric applications.